The hypothesis that the higher respiratory exchange ratio during exercise in niddm reflects a metabolic shift towards an earlier and greater reliance on carbohydrate metabolism at constant load work loads as compared to nondiabetic controls. Exercise performance is impaired both at maximal land submaximal levels in persons with niddm, especially women, even in the absence of complications. One finding in particular was that VO2/ work load was lower in niddm compared to control during exercise, a finding which in certain cardiorespiratory disease states has been interpreted to suggest impaired oxygen delivery. In addition, we found that the respiratory exchange ratio was higher in niddm that in nondiabetics for submaximal work loads.